The Phoenix Suns dominated the N.B.A. regular season and set a franchise record for wins with one goal in mind: return to the N.B.A. finals for a second straight year — and win the championship this time.

They won’t even get a chance to try.

The top-seeded Suns lost to the No. 4-seeded Dallas Mavericks, 123-90, in Game 7 of the teams’ Western Conference semifinals series on Sunday in Phoenix. Dallas, which led by 30 points at halftime, will face Golden State on Wednesday in San Francisco in Game 1 of the conference finals.

Sunday’s game between the Suns and Mavericks was the first time the road team won a game in the series. The Suns had beaten the Mavericks by an average of 19 points per game in the previous three games in Phoenix. Dallas led by as many as 46 on Sunday.

For Phoenix, the loss ended a season that began ominously when the N.B.A. started investigating Robert Sarver, the owner of the team and the W.N.B.A.’s Phoenix Mercury, after current and former employees accused him of racist, sexist and otherwise inappropriate behavior. The results of the investigation have not yet been announced.

On the court, it was the Suns’ most promising season in 12 years. They were nearly unbeatable, going 64-18 during the regular season.

They lost three out of their first four games, but then went on an 18-game winning streak that included two wins over the Mavericks and one over Golden State. The Suns set the franchise single-season wins record with their 63rd victory, which came against the Los Angeles Lakers in a game that knocked the Lakers out of playoff contention.

The Suns were led by two All-Star guards: the 25-year-old Devin Booker, and the 37-year-old Chris Paul, in his 17th N.B.A. season. Last season marked the first time Paul had ever been to the N.B.A. finals.

Suns wing Mikal Bridges finished second in voting for the league’s Defensive Player of the Year Award, and Monty Williams was named coach of the year.

As the playoffs approached, Williams worked to balance his desire to rest players, heading into what they expected to be a long playoff run, with a need to keep them playing for momentum heading into the postseason.

They lost four of their last six games, but still entered the playoffs as heavy favorites to advance easily through their side of the bracket as the No. 1 seed.

Right from the start their path was rockier than expected. They took six games to beat the New Orleans Pelicans, who had sneaked into the playoffs through the play-in tournament after having the ninth-best record in the Western Conference.

The Suns have the most wins and finals appearances of any N.B.A. team that has not won a championship.

Last season, Phoenix made its third trip to the finals and fell to the Milwaukee Bucks, losing four consecutive times after winning the first two games.

The Bucks also were eliminated from the playoffs on Sunday, losing a Game 7 to the Boston Celtics, who will face the top-seeded Miami Heat in the Eastern Conference finals starting Tuesday.