Twenty years ago Britney Spears recorded “I’m Not A Girl, Not Yet A Woman.” A goopy piano line and a karaoke-bar drum machine preset signal its seriousness. She and producer Max Martin wanted a ballad manifesto for Crossroads, a road-trip film about being Britney Spears but with Dan Aykroyd as the father resisting her maturation. For their idea of sensitive lyrics they tapped Dido Armstrong, whose “Thank You” turned into a massive hit in its own right after Eminem’s fan-ambivalent “Stan” interpolated its chorus. Recorded in Stockholm with Martin’s crew, “I’m Not A Girl” boasts a solid Spears performance; she sings the words in her thin, whiskery, fey timbre. She sounds like neither girl nor woman: She sounds as if the pitch-altered Prince of “U Got The Look” had imitated a senior citizen. The result is weird, powerful, and listenable — check out that key change in its last third, where she goes for it — if not quite a triumph.