Gate-controlled conductance of superconducting NbN nanowires: coherent quantum phase-slip or Coulomb blockade?

Coherent quantum phase-slips (CQPS) are expected to lead to a blockade of dc conduction in sufficiently narrow superconducting nanowires below a certain critical voltage. We present measurements of NbN nanowires in which not only is a critical voltage observed, but also in which this critical voltage may be tuned using a side-gate electrode. The critical voltage varies periodically as the applied gate voltage is varied. While the observations are qualitatively as expected for quantum interference between CQPS elements, the period of the tuning is orders of magnitude larger than expected on the basis of simple capacitance considerations. Furthermore, two significant abrupt changes in the period of the variations during measurements of one nanowire are observed, an observation which constrains detailed explanations for the behaviour. The plausibility of an explanation assuming that the behaviour arises from granular Josephson junctions in the nanowire is also considered.