1. Institute of Physics, Johannes Gutenberg-University, D-55 128 Mainz, Germany
2. Department of Physics, University of Gothenburg, SE-412 96 Gothenburg, Sweden
3. CERN, 1211 Geneva, Switzerland
4. Department of Physics, University of Tennessee, Knoxville, Tennessee 37 996, USA
5. Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden