1. Department of Material and Life Science, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan, Nanoscale Quantum Conductor Array Project, ICORP, Japan Science and Technology Agency (JST), 4-1-8 Honmachi, Kawaguchi, Saitama 332-0112, Japan, Harima Institute, The Institute of Physical and Chemical Research (RIKEN), 1-1-1 Kokuto, Mikazuki, Hyogo 679-5148, Japan, and Nanomaterials Laboratory, National Institute of Material Science (NIMS), 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan