  We estimate the depairing current of superconducting nanowire single photon
detectors (SNSPDs) by studying the dependence of the nanowires kinetic
inductance on their bias current. The kinetic inductance is determined by
measuring the resonance frequency of resonator style nanowire coplanar
waveguides both in transmission and reflection configurations. Bias current
dependent shifts in the measured resonant frequency correspond to the change in
the kinetic inductance, which can be compared with theoretical predictions. We
demonstrate that the fast relaxation model described in the literature
accurately matches our experimental data and provides a valuable tool for
direct determination of the depairing current. Accurate and direct measurement
of the depairing current is critical for nanowire quality analysis, as well as
modeling efforts aimed at understanding the detection mechanism in SNSPDs.
