1. D. Shahar, D. C. Tsui, M. Shayegan, Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA.
2. E. Shimshoni, Department of Physics and the Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
3. S. L. Sondhi, Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.