Violation of Ioffe-Regel condition but saturation of resistivity of the   high Tc cuprates
  We demonstrate that the resistivity data of a number of high Tc cuprates, in particular La(2-x)SrxCuO4, are consistent with resistivity saturation, although the Ioffe-Regel condition is strongly violated. By using the f-sum rule together with calculations of the kinetic energy in the t-J model, we show that the saturation resistivity is unusually large. This is related to the strong reduction of the kinetic energy due to strong correlation effects. The fulfilment of the Ioffe-Regel condition for conventional transition metal compounds is found to be somewhat accidental. 
M. Calandra and O. Gunnarsson (Max-Planck-Institut fur   Festkorperforschung, Stuttgart, Germany)