  We present electrical transport data for single-crystalline Co$_2$FeSi which
provide clear-cut evidence that this Heusler compound is truly a half-metallic
ferromagnet, i.e. it possesses perfect spin-polarization. More specifically,
the temperature dependence of $\rho$ is governed by electron scattering off
magnons which are thermally excited over a sizeable gap $\Delta\approx 100 K$
($\sim 9 meV$) separating the electronic majority states at the Fermi level
from the unoccupied minority states. As a consequence, electron-magnon
scattering is only relevant at $T\gtrsim\Delta$ but freezes out at lower
temperatures, i.e., the spin-polarization of the electrons at the Fermi level
remains practically perfect for $T\lesssim\Delta$. The gapped magnon population
has a decisive influence on the magnetoresistance and the anomalous Hall effect
(AHE): i) The magnetoresistance changes its sign at $T\sim 100 K$, ii) the
anomalous Hall coefficient is strongly temperature dependent at $T\gtrsim 100
K$ and compatible with Berry phase related and/or side-jump electronic
deflection, whereas it is practically temperature-independent at lower
temperatures.
