K.M.A. is very grateful to David Grier, Hellmut Fritzsche, and Stephan Koehler for helpful discussions and advice and to Cynthia Hunt for contributions to the setup and acknowledges the support of an AT&T Bell Laboratories Ph.D. Fellowship while conducting this research. This work was supported in part by the MRSEC Program of the National Science Foundation under Award Number DMR-9400379.