  We report on experimental study of the effect of disorder on electronic
parameters and inelastic scattering mechanisms in ultrathin superconducting NbN
films, which are commonly used in single-photon detectors. An increase in
disorder in the studied 2.5 nm thick NbN films characterized by Ioffe-Regel
parameter from 6.3 to 1.6 is accompanied by a decrease in the critical
temperature $T_c$ from 11.5 K to 3.4 K. By measuring magnetoconductance in the
range from $T_c$ to $\sim3T_c$, we extract the inelastic scattering rates of
electrons, including electron-phonon (e-ph) scattering rates
$\tau_{e-ph}^{-1}$. We observe that $\tau_{e-ph}^{-1}$ and their temperature
dependencies are insensitive to disorder that is not described by the existing
models of the e-ph scattering in disordered metals and can be due to the
presence of weakly disordered metal grains. As the temperature decreases the
temperature dependence $\tau_{e-ph}^{-1}$ changes from $T^3$ to $T^2$, which
can be result of a decrease in the dimension of the phonons involved in the
e-ph scattering. The obtained values of material parameters of ultrathin NbN
films can be useful for optimization of performance of NbN-based electronic
devices.
