1. Quantronics group, Service de Physique de l'État Condensé, DRECAM, CEA‐Saclay, 91191 Gif‐sur‐Yvette, France
2. Current address: Department of Physics, Stanford University, Stanford, California 94305, USA
3. Permanent address: Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824‐2320, USA
4. Nanoelectronics group, Service de Physique de l'État Condensé, DRECAM, CEA‐Saclay, 91191 Gif‐sur‐Yvette, France
5. Institut für Theoretische Physik, Universität Ulm, Albert‐Einstein‐Allee 11, 89069 Ulm, Germany