1. a Xerox Webster Research Center , 800 Phillips Rd., W114, Webster, NY, 14580, U.S.A.
2. f SIGNAL UOP Research Center , 50 UOP Plaza, DesPlaines, IL, 60016-6787
3. g AT&T Technologies, Engineering Research Center , P.O. Box 900, Princeton, NJ, 08540
4. b Dept. of Physics , Univ. of Florida , Gainesville, FL, 32611, U.S.A.
5. c Laboratoire de Physique des Solides , Universite Paris-Sud , 91405, Orsay, FRANCE
6. d L.C.R. , Thomson CSF, 91401, Orsay, FRANCE
7. e Dept. of Physics , SUNY at Stony Brook , Stony Brook, NY, 11794, U.S.A.