  We report results on electronic transport properties of liquid Fe-S alloys at
conditions of planetary cores, computed by first-principle techniques in the
Kubo-Greenwood formalism. We describe a combined effect of resistivity
saturation due to temperature, compression, and chemistry by comparing the
electron mean free path from the Drude response of optical conductivity to the
mean interatomic distance. At high compression and high sulfur concentration
the Ioffe-Regel condition is satisfied, and the temperature coefficient of
resistivity changes sign from positive to negative. We show that this happens
due to a decrease of the $d$-density of states at the Fermi level in response
to thermal broadening.
