1. Centre de Recherche sur les Très Basses Témperatures, C.N.R.S., BP 166X, F-38042 Grenoble, France
2. Institute for Physical and Theoretical Chemistry, University of Tübingen, Morgenstelle 8, D-7400 Tübingen, Germany
3. Physical Institute, University of Tübingen, Morgenstelle 14, D-7400 Tübingen, Germany
4. Physics Institute, University of Zurich, Schönberggasse 9, CH-8001 Zurich, Switzerland