[1] Natl Phys Lab, Teddington TW11 0LW, Middx, England
[2] Phys Tech Bundesanstalt, D-10587 Berlin, Germany
[3] Lawrence Berkeley Natl Lab, Berkeley, CA USA
[4] Univ Florida, Dept Phys, Gainesville, FL 32611 USA
[5] Inst Natl Metrol, Lab Natl Essais, F-93210 St Denis, France
[6] Leiden Univ, Kamerlingh Onnes Lab, NL-2300 AR Leiden, Netherlands