  The optical properties of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ have been
determined in the normal state for a number of temperatures over a wide
frequency range. Two Drude terms, representing two groups of carriers with
different scattering rates ($1/\tau$), well describe the real part of the
optical conductivity, $\sigma_{1}(\omega)$. A "broad" Drude component results
in an incoherent background with a $T$-independent $1/\tau_b$, while a "narrow"
Drude component reveals a $T$-linear $1/\tau_n$ resulting in a resistivity
$\rho_n \equiv 1/\sigma_{1n}(\omega\rightarrow 0)$ also linear in temperature.
An arctan($T$) low-frequency spectral weight is also a strong evidence for a
$T$-linear 1/$\tau$. Comparison to other materials with similar behavior
suggests that the $T$-linear $1/\tau_n$ and $\rho_n$ in
Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ originate from scattering from spin
fluctuations and hence that an antiferromagnetic quantum critical point is
likely to exist in the superconducting dome.
