  The anomalous charge transport observed in some strongly correlated metals
raises questions as to the universal applicability of Landau Fermi liquid
theory. The coherence temperature $T_{FL}$ for normal metals is usually taken
to be the temperature below which $T^2$ is observed in the resistivity. Below
this temperature, a Fermi liquid with well-defined quasiparticles is expected.
However, metallic ruthenates in the Ruddlesden-Popper family, frequently show
non-Drude low-energy optical conductivity and unusual $\omega/T$ scaling,
despite the frequent observation of $T^2$ dc resistivity. Herein we report
time-domain THz spectroscopy measurements of several different high-quality
metallic ruthenate thin films and show that the optical conductivity can be
interpreted in more conventional terms. In all materials, the conductivity has
a two-Drude peak lineshape at low temperature and a crossover to a one-Drude
peak lineshape at higher temperatures. The two-component low-temperature
conductivity is indicative of two well-separated current relaxation rates for
different conduction channels. We discuss three particular possibilities for
the separation of rates: (a) Strongly energy-dependent inelastic scattering;
(b) an almost-conserved pseudomomentum operator that overlaps with the current,
giving rise to the narrower Drude peak; (c) the presence of multiple conduction
channels that undergoes a crossover to stronger interband scattering at higher
temperatures. None of these scenarios require the existence of exotic
quasiparticles. The results may give insight into the possible significance of
Hund's coupling in determining interband coupling in these materials. Our
results also show a route towards understanding the violation of Matthiessen's
rule in this class of materials and deviations from the "Gurzhi" scaling
relations in Fermi liquids.
