1. Quantum- and Nanoelectronics Lab, Department of Physics, University of Basel, 4056 Basel, Switzerland
2. MESA+ Institute for Nanotechnology University of Twente, 7500 AE Enschede, The Netherlands
3. Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
4. Swiss Nanoscience Institute, University of Basel, 4056 Basel, Switzerland