  The Hall effect and resistivity of the carrier doped magnetic semiconductor
Fe$_{1-x}$Co$_x$S$_2$ were measured for $0\le x \le 0.16$, temperatures between
0.05 and 300 K, and fields of up to 9 T. Our Hall data indicate electron charge
carriers with a density of only 10 to 30% of the Co density of our crystals.
Despite the previous identification of magnetic Griffiths phase formation in
the magnetic and thermodynamic properties of this system for the same range of
$x$, we measure a temperature independent resistivity below 0.5 K indicating
Fermi liquid-like transport. We also observe no indication of quantum
corrections to the conductivity despite the small values of the product of the
Fermi wave vector and the mean-free-path, $1.5 \le k_F\ell \le 15$, over the
range of $x$ investigated. This implies a large inelastic scattering rate such
that the necessary condition for the observation of quantum contributions to
the carrier transport, quantum coherence over times much longer than the
elastic scattering time, is not met in our samples. Above 0.5 K we observe a
temperature and magnetic field dependent resistivity that closely resembles a
Kondo anomaly for $x$ less than that required to form a long range magnetic
state, $x_c$. For $x>x_c$, the resistivity and magnetoresistance resemble that
of a spin glass with a reduction of the resistivity by as much as 35% in 5 T
fields. We also observe an enhancement of the residual resistivity ratio by
almost a factor of 2 for samples with $x\sim x_c$ indicating temperature
dependent scattering mechanisms beyond simple carrier-phonon scattering. We
speculate that this enhancement is due to charge carrier scattering from
magnetic fluctuations which contribute to the resistivity over a wide
temperature range.

Strongly Correlated Electrons;Materials Science