GOLETA, Calif. — A 2-mile-long oil slick mysteriously materialized off the Southern California coast on Wednesday, but unlike a disastrous spill in the same area two months ago it didn’t send any damaging crude ashore and was expected to dissipate on its own. The Coast Guard was still attempting to determine the source of the slick, which was first observed just off the beach in the college town of Goleta on Wednesday morning. “I’m not sure when we’ll find out,” said Petty Officer 1st Class Sondra-Kay Kneen.