1. Faculty of Science and Technology, Keio University, 3‐14‐1 Hiyoshi, Yokohama 223, Japan
2. Kyusyu Institute of Design, Shiobaru, Minami‐ku, Fukuoka 815, Japan
3. NTT LSI Laboratories, Morinosato Wakamiya, Atsugi‐si, Kanagawa 243‐01, Japan
4. SEH R&D Center, Shin‐Etsu Handotai Co., Ltd., 2‐13‐1 Isobe, Annaka, Gunma 379‐01, Japan