1. D. R. Stewart, Department of Applied Physics, Stanford University, Stanford, CA 94305, USA.
2. D. Sprinzak, Department of Physics, Stanford University, Stanford, CA 94305, USA, and Braun Center for Submicron Research, Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
3. C. M. Marcus, Department of Physics, Stanford University, Stanford, CA 94305, USA.
4. C. I. Duruöz and J. S. Harris Jr., Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA.