  Thorough spectral study of the intrinsic single-photon detection efficiency
in superconducting TaN and NbN nanowires with different widths shows that the
experimental cut-off in the efficiency at near-infrared wavelengths is most
likely caused by the local deficiency of Cooper pairs available for current
transport. For both materials the reciprocal cut-off wavelength scales with the
wire width whereas the scaling factor quantitatively agrees with the hot-spot
detection models. Comparison of the experimental data with vortex-assisted
detection scenarios shows that these models predict a stronger dependence of
the cut-off wavelength on the wire width.
