By Larry Blumenfeld
Updated March 30, 2007 04:00 AM
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Guitarist Pat Metheny and pianist Brad Mehldau both wield powerful, genre-defying influence: Each is a distinctive soloist whose greatest strength is nonetheless a collaborative urge. The four duets and seven tracks on Quartet with Mehldau’s fine triomates (bassist Larry Grenadier, drummer Jeff Ballard) move gracefully from gentle swing to hard-edged improvisation. A-