Eighteen-year-old folkie Laura Marling aims to join a crowded field of young, female British songwriters that includes Lily Allen, Kate Nash, Adele and Amy Winehouse. On her full-length debut, "Alas, I Cannot Swim," Marling does more than keep her head above water, buoyed by rapid-fire confessional lyrics and a voice that can hang with any of her British sisters. Or brothers. She joins us for a live performance.