  We present calculations of electrical resistivity for expanded boron,
aluminum, titanium and copper plasmas using the Ziman formulation in the
framework of the average-atom model. Our results are compared to experimental
data, as well as with other theoretical calculations, relying on the Ziman and
Kubo-Greenwood formulations, and based on average-atom models or
quantum-molecular-dynamics simulations. The impact of the definition of
ionization, paying a particular attention to the consistency between the latter
and the perfect free electron gas assumption made in the formalism, is
discussed. We propose a definition of the mean ionization generalizing to
expanded plasmas the idea initially put forward for dense plasmas, consisting
in dropping the contribution of quasi-bound states from the ionization due to
continuum ones. It is shown that our recommendation for the calculation of the
quasi-bound density of states provides the best agreement with measurements.
