This work was supported in part by the Advanced Research Projects Agency of the Department of Defense under contracts DABT63-92-C-0026 and F30602-95-C-0136, by the Department of Energy grant DE-FG03-94ER25206, and by the National Science Foundation grants CCR-9210260, CDA-8722788, and CDA-9401156. The information presented here does not necessarily reflect the position or the policy of the Government and no official endorsement should be inferred.