1. Joseph Henry Laboratories and Department of Physics, Princeton University, Princeton, NJ 08544, U.S.A.
2. Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A.
3. University of California, Irvine, CA 92697, U.S.A.
4. Kamerlingh Onnes Laboratory, Leiden University, 2300 RA Leiden, The Netherlands