Stocks quickly trimmed their opening gains Wednesday, turning narrowly mixed in early trading. The Nasdaq defended a 0.4% gain. The S&P 500 held on to a fractional advance. The Dow Jones industrial average dipped 0.1%. Small caps outpaced the market, with the Russell 2000 climbing 0.7%. Volume rose 10% on the NYSE and slipped 36% on the Nasdaq compared with the same time Tuesday.