  The absence of resistivity saturation in many strongly correlated metals,
including the high-temperature superconductors, is critically examined from the
viewpoint of optical conductivity measurements. Coherent quasiparticle
conductivity, in the form of a Drude peak centred at zero frequency, is found
to disappear as the mean free path (at $\omega$ = 0) becomes comparable to the
interatomic spacing. This basic loss of coherence at the so-called
Mott-Ioffe-Regel (MIR) limit suggests that the universality of the MIR
criterion is preserved even in the presence of strong electron correlations. We
argue that the shedding of spectral weight at low frequencies, induced by
strong correlation effects, is the primary origin of the extended positive
slope of the resistivity to high temperatures observed in all so-called "bad
metals". Moreover, in common with those metals which exhibit resistivity
saturation at high temperatures, the scattering rate itself, as extracted from
optical spectra, saturates at a value consistent with the MIR limit. We
consider possible implications that this ceiling in the scattering rate may
have for our understanding of transport within a wide variety of bad metals and
suggest a better method for analysing their optical response.
