Geerligs, Peters, and Mooij Reply:The clear distinction between mutual and self-capacitance that Ferrell and Mirhashem l make is significant for quantitative comparison of our experiments with theories of phase transitions. In our Letter 2 the transition at x around 1 was quoted 3 as an approximate threshold. As such the distinction is less relevant since both choices of capacitance yield comparable results. In a mean-field approximation, e.g., Efetov finds for self-charging a critical x equal to l. 3 Doniach arrives, for mutual capacitance, at x=2. 3 Recently, Eckern and Schon performed a mean-field treatment for arbitrary mutual and self-capacitance. 4 They also find a critical x of order 1 for mutual capacitance much larger than self-capacitance.We agree that for a more reliable check of theory with experiments smaller junctions would be helpful but of course their fabrication is nontrivial. Among other present uncertainties are the effect of additional elements in the capacitance matrix, and the question of the appropriate "tunneling horizon" for the determination of C e ff. 5 This last item can make a difference of about a factor of 3 in the experimental determination of the junction capacitance.

We describe the behavior of charges in two-dimensional arrays of normal-metal tunnel junctions with very small capacitance. A Kosterlitz-Thouless-Berezinskii phase transition with unbinding of chargeanticharge pairs occurs at a transition temperature of about 7V sss e 2 /SnCkBy with C the junction capacitance. We calculate the influence of tunneling conductance; T c is reduced with increasing conductance and no transition occurs for junction conductance above (14 kft) -1 . In the superconducting state a similar transition occurs at a 4 times higher T c . We present the first experimental results on the conductive transition of an array in the normal and superconducting states.

Two-dimensional arrays of very-small-capacitance Josephson junctions have been studied. At low temperatures the arrays show a transition from superconducting to insulating behavior when the ratio of charging energy to Josephson-coupling energy exceeds the value 1. Insulating behavior coincides with the occurrence of a charging gap inside the BCS gap, with an S-shaped I-V characteristic. This so far unobserved S shape is predicted to arise from macroscopic quantum coherent effects including Bloch oscillations.

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We report measurements on submicron metal-oxide-semiconductor field effect transistors equipped with a gate on three sides of the channel. At room temperature, a strong suppression of short-channel effects has been achieved for the narrowest channels. At liquid helium temperatures, the same devices exhibit clear conductance oscillations in the subthreshold regime, indicating that a quantum dot has formed in the disordered channel.