California Atty. Gen. John K. Van de Kamp, at a meeting in San Diego of law enforcement officials from the Southwest and Mexico, said Mexican citizens charged with drug violations under California law should be returned to Mexico for trial "to improve the quality of cooperation" between U.S. and Mexican authorities. Van de Kamp said his office sent 19 cases involving violent crimes to Mexican federal prosecutors for trial last year under a provision of Mexican law that allows for such transfers. "Now we'd like to try sending narcotics cases, too," he said. "If we could get a conviction of a major peddler it would send a message to others. It is also a very good device to improve the quality of cooperation."