  We report measurements on single-electron pumps, consisting of two metallic
islands formed by three tunnel junctions in series. We focus on the
linear-response conductance as a function of gate voltage and temperature of
three samples with varying system parameters. In all cases, strong quantum
fluctuation phenomena are observed by a log(k_B T/(2 E_co)) reduction of the
maximal conductance, where E_co measures the coupling strength between the
islands. The samples display a rich phenomenology, culminating in a
non-monotonic behavior of the maximal conductance as a function of temperature.
