There were a lot of different words used to describe the Nets’ 111-86 meltdown against the Spurs last night at the Barclays Center. Coach P.J. Carlesimo and Joe Johnson both called it “unacceptable.” Brook Lopez talked about a lack of energy, while Deron Williams blamed it on bad habits. The truth is the Nets are a soft team: soft physically and soft mentally. It’s why the Spurs, who trailed by 10 points after the first quarter, authored a systematic beat-down in the second half, when they outscored the Nets 60-29. The Spurs did this without Tim Duncan and Manu Ginobili, who missed the game with injuries. Tony Parker (29 points, 11 assists) was all San Antonio (40-12) needed to hand the Nets (29-22) their sixth loss in their last nine games. “When things start to go bad, instead of fighting back we kind of hang our heads,” Williams said. “It’s just a mentality that we have. We’ve got to get better at that.” Being soft is not a word any professional athlete wants to be called. But the Nets were a soft team last night, a team still struggling to learn how to win. Maybe it’s because they’re a brand new team in a brand new building guided by an interim head coach. Maybe it’s because the bench hasn’t offered much support in terms of passion or productivity. Maybe it’s because their so-called superstars aren’t playing like superstars. Whatever the reason, inconsistency has been the hallmark of their existence this season. And now the team is starting to show signs it lacks a backbone. “We’ve got to be able to respond,” forward Gerald Wallace said. “When things go bad, we kind of drop our heads, and instead of pulling together as a team we go in five different directions out on the court, and it shows.” There was no fight in the Nets. They started aggressive and confident and led 35-25 after the first quarter. But the Spurs tightened their defense and increased their resolve and it proved more than enough to overwhelm the Nets.