The conductance of linear arrays of two and three normal-metal small tunnel junctions is studied for bias voltages V below the Coloumb-blockade threshold. At low temperature, we find evidence for macroscopic quantum tunneling of the electric charge (q-MQT) through the Coulomb energy barrier. For double junctions the tunneling rate scales as ${\mathit{V}}^3$, and approximately as the product of the junction conductances, as predicted by the theory of inelastic q-MQT.

• Received 20 July 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.3037