Strong suppression of the resistivity near the superconducting transition in narrow microbridges in external magnetic fields

We have investigated a series of superconducting bridges based on homogeneous amorphous WSi and MoSi films, with bridge widths $w$ ranging from 2 to $1000\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ and film thicknesses $d\ensuremath{\sim}4\ensuremath{-}6$ and 100 nm. Upon decreasing the bridge widths below the respective Pearl lengths, we observe in all cases distinct changes in the characteristics of the resistive transitions to superconductivity. For each of the films, the resistivity curves $R(B,T)$ separate at a well-defined and field-dependent temperature ${T}^*(B)$ with decreasing the temperature, resulting in a dramatic suppression of the resistivity and a sharpening of the transitions with decreasing bridge width $w$. The associated excess conductivity in all the bridges scales as $1/w$, which may suggest either the presence of a highly conducting region that is dominating the electric transport, or a change in the vortex dynamics in narrow enough bridges. We argue that this effect can only be observed in materials with sufficiently weak vortex pinning.