Acknowledgements—The authors thank J. P. Wolfe and Kent McCormick for useful discussions. This work was funded at Harvard by ONR N00014-95-1-0104;
!text type='JS']JS[!/text]EP N00014-89-J-1023;
NSF DMR-95-01438 and the MRSEC program of the National Science Foundation under DMR-94-00396;
and at UCSB by AFOSR grant F49620-94-1-0158;