  Many transition metal compounds show saturation of the resistivity at high
temperatures, T, while the alkali-doped fullerenes and the high-Tc cuprates are
usually considered to show no saturation. We present a model of transition
metal compounds, showing saturation, and a model of alkali-doped fullerenes,
showing no saturation. To analyze the results we use the f-sum rule, which
leads to an approximate upper limit for the resistivity at large T. For some
systems and at low T, the resistivity increases so rapidly that this upper
limit is approached for experimental T. The resistivity then saturates. For a
model of transition metal compounds with weakly interacting electrons, the
upper limit corresponds to a mean free path consistent with the Ioffe-Regel
condition. For a model of the high Tc cuprates with strongly interacting
electrons, however, the upper limit is much larger than the Ioffe-Regel
condition suggests. Since this limit is not exceeded by experimental data, the
data are consistent with saturation also for the cuprates. After "saturation"
the resistivity usually grows slowly. For the alkali-doped fullerenes,
"saturation" can be considered to have happened already for T=0, due to
orientational disorder. For these systems, however, the resistivity grows so
rapidly after "saturation" that this concept is meaningless. This is due to the
small band width and to the coupling to the level energies of the important
phonons.
