1. Laboratory for Mesoscopic Systems, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland
2. Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
3. Department of Physics and Astronomy, Uppsala University, Box 516, SE-75120 Uppsala, Sweden
4. Helmholtz-Zentrum Dresden-Rossendorf, Dresden 01328, Germany