[1] Bell Labs, Lucent Technol, Murray Hill, NJ 07974 USA
[2] CALTECH, Dept Appl Phys, Pasadena, CA 91125 USA
[3] Harvard Univ, Div Engn & Appl Sci, Cambridge, MA 02138 USA
[4] New Jersey Nanotechnol Consortium, Murray Hill, NJ 07974 USA
[5] Univ Paris 11, Inst Elect Fondamentale, F-91405 Orsay, France
[6] Princeton Univ, Dept Elect Engn, Princeton, NJ 08544 USA