  The fabrication of high-quality thin superconducting films is essential for
single-photon detectors. Their device performance is crucially affected by
their material parameters, thus requiring reliable and nondestructive
characterization methods after the fabrication and patterning processes.
Important material parameters to know are the resistivity, superconducting
transition temperature, relaxation time of quasiparticles, and uniformity of
patterned wires. In this work, we characterize micro-patterned thin NbN films
by using transport measurements in magnetic fields. We show that from the
instability of vortex motion at high currents in the flux-flow state of the
$IV$ characteristic, the inelastic life time of quasiparticles can be
determined to be about 2 ns. Additionally, from the depinning transition of
vortices at low currents, as a function of magnetic field, the size
distribution of grains can be extracted. This size distribution is found to be
in agreement with the film morphology obtained from scanning electron
microscopy and high-resolution transmission electron microscopy images.
