  Fermi's golden rule underpins the investigation of mobile carriers
propagating through various solids, being a standard tool to calculate their
scattering rates. As such, it provides a perturbative estimate under the
implicit assumption that the effect of the interaction Hamiltonian which causes
the scattering events is sufficiently small. To check the validity of this
assumption, we present a general framework to derive simple validity criteria
in order to assess whether the scattering rates can be trusted for the system
under consideration, given its statistical properties such as average size,
electron density, impurity density et cetera. We derive concrete validity
criteria for metallic nanowires with conduction electrons populating a single
parabolic band subjected to different elastic scattering mechanisms:
impurities, grain boundaries and surface roughness.
