  This paper has been withdrawn by the authors. The authors have now written
two separate papers, one theoretical (A. Smerzi, A. Trombettoni, P. G.
Kevrekidis, and A. R. Bishop Phys. Rev. Lett. 89, 170402 (2002)) and one
experimental (F. S. Cataliotti, L. Fallani, F. Ferlaino, C. Fort, P. Maddaloni,
and M. Inguscio New Journal of Physics 5, 71 (2003)).
