1. ULSI Laboratory, Mitsubishi Electric Company, 4‐1 Mizuhara, Itami 664, Japan
2. Research Institute for Scientific Measurements, Tohoku University, 2‐1‐1 Katahira, Aoba‐ku, Sendai 980‐77, Japan
3. Research Institute of Electrical Communication, Tohoku University, 2‐1‐1 Katahira, Aoba‐ku, Sendai 980‐77, Japan