  We perform, as a function of uniaxial stress, an optical-reflectivity
investigation of the representative 'parent' ferropnictide BaFe$_2$As$_2$ in a
broad spectral range, across the tetragonal-to-orthorhombic phase transition
and the onset of the long-range antiferromagnetic order (AFM). The infrared
response reveals that the $dc$ transport anisotropy in the orthorhombic AFM
state is determined by the interplay between the Drude spectral weight and the
scattering rate, but that the dominant effect is clearly associated with the
metallic spectral weight. In the paramagnetic tetragonal phase, though, the
$dc$ resistivity anisotropy of strained samples is almost exclusively due to
stress-induced changes in the Drude weight rather than in the scattering rate,
definitively establishing the anisotropy of the Fermi surface parameters as the
primary effect driving the $dc$ transport properties in the electronic nematic
state.
