The tunneling characteristics of a two-junction system created with extremely small metal particles is studied in the voltage region within the Coulomb blockade. We observe a conductance orders of magnitude lower than outside the blockade, which matches predictions for a superposition of elastic and inelastic macroscopic quantum tunneling of the charge variable Q.

• Received 2 December 1991

DOI:https://doi.org/10.1103/PhysRevLett.68.3228