1. Bell Laboratories, Lucent Technologies, Murray Hill, NJ 07974, USA.
2. Department of Applied Physics, California Institute of Technology, Pasadena, CA 91125, USA.
3. Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA.
4. New Jersey Nanotechnology Consortium, Murray Hill, NJ 07974, USA.
5. Institut d'Electronique Fondamentale, Bât.220, Université Paris-Sud, 91405 Orsay, France.