1. Exxon Research and Engineering, Route 22 East, Annandale, New Jersey 08801, USA
2. Department of Physics, Haverford College, Haverford, Pennsylvania 19041, USA
3. Hochfeld Magnetlabor, Max Planck Institut für Festkörperforschung 166x, F-98042 Grenoble-Cede, France
4. Centre de Recherches sur les Très Basses Températures, CNRS, F-38042 Grenoble-Cedex, France
5. Department of Physics, Princeton University, Princeton, New Jersey 07895, USA