1. Department of Physics, University of California at Santa Barbara, Santa Barbara California 93106, United States
2. Protemics GmbH, 52074 Aachen, Germany
3. Department of Physics, University of California at San Diego, La Jolla, California 92093, United States
4. Nanofabrication Facility, Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93106, United States