  We discuss the charge carrier dynamics of the heavy-fermion compound
CeCoIn$_5$ in the metallic regime measured by means of quasi-optical THz
spectroscopy. The transmittance of electromagnetic radiation through a
CeCoIn$_5$ thin film on a dielectric substrate is analyzed in the
single-particle Drude framework. We discuss the temperature dependence of the
electronic properties, such as the scattering time and dc-conductivity and
compare with transport measurements of the sheet resistance. Towards low
temperatures, we find an increasing mismatch between the results from transport
and Drude-analyzed optical measurements and a growing incapability of the
simple single-particle picture describing the charge dynamics, likely caused by
the evolving heavy-fermion nature of the correlated electron system.
