1. Department of Electrical and Computer Engineering, University of California, Davis, CA 95616
2. Permanent address: Department of Physics, Faculty of Sciences, University Sidi Mohamed Ben Abdellah, BP 1796, Atlas-Fes, Morocco.
3. IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120
4. Max-Planck-Institute for Polymer Research, 55128 Mainz, Germany
5. Also with the Frontier Research Program, The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-01, Japan.