1. Departments of Chemistry and Physics, McGill University, 801 Sherbrooke St. W. Montreal, Quebec H3A 2K6, Canada
2. Department of Cell Biology, University of Virginia, PO Box 800732, Charlottesville, VA 22908, USA
3. Department of Physics, Colorado School of Mines, 1523 Illinois Street, Golden, CO 80401, USA
4. Departments of Neurosciences and Bioengineering, and National Center for Microscopy and Imaging Research, University of California, San Diego, CA 92093, USA