1. The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
2. Department of Applied Physics, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
3. H. H. Wills Physics Laboratory, The University of Bristol, Bristol BS8-1TL, U.K.
4. Research Institute for Solid State Physics and Optics, H-1525 Budapest, P.O.B. 49, Hungary