1. Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113–0033, Japan.
2. Nippon Telegraph and Telephone Basic Research Laboratories, NTT Corporation, Atsugi-shi, Kanagawa 243–0198, Japan.
3. Institute for Microstructural Sciences M23A, National Research Council of Canada, Ottawa, Ontario K1A 0R6, Canada.
4. Exploratory Research for Advanced Technology Mesoscopic Correlation Project, Japan Science and Technology, Atsugi-shi, Kanagawa 243–0198, Japan.