  In systems with reduced dimensions quantum fluctuations have a strong
influence on the electronic conduction, even at very low temperature. In
superconductors this is especially interesting, since the coherent state of the
superconducting electrons is strongly interacting with these fluctuations and
therefore is a sensitive tool to study them. In this paper, we report on
comprehensive measurements of superconducting nanowires in the quantum phase
slip regime. Using an intrinsic electromigration process, we have developed a
method to lower the resistance of lithographically fabricated highly resistive
nanowires in situ and in small consecutive steps. At low temperature we observe
critical (Coulomb) blockade voltages and superconducting critical currents,
depending on the nanowire's normal-state resistance, in good agreement with
theoretical models. Between these two regimes, we find a continuous transition
displaying a nonlinear metallic-like behavior. The reported intrinsic
electromigration technique is not limited to low temperatures as we find a
similar change in resistance that spans over three orders of magnitude also at
room temperature. Aside from superconducting quantum circuits, such a technique
to reduce the resistance may also have applications in modern electronic
circuits.
