The conductivity of plasma can be related to its microscopic properties using the Drude model

\sigma \approx \frac{n q^2}{m \omega}

where n is the number density, q is the elementary charge, m is the mass of the electron and \omega is the collision rate. The latter is given by

 \omega \approx n \sigma v

where  \sigma is the cross section for collisions and  v is the thermal velocity. To calculate the cross section, we note the impact parameter below which a scattering angle between an electron and a proton will be of order unity

b \approx \frac{q^2}{m v^2}


 \sigma \approx b^2 \approx \frac{q^4}{m^2 v^4}

Finally, the resistivity is given by

\eta = \frac{1}{\sigma} \approx \frac{m}{n q^2} n v \frac{q^4}{m^2 v^4} \approx \frac{q^2}{m v^3} \approx \frac{q^2 \sqrt{m}}{\left(k T \right )^{3/2}}

where  T \, is the temperature and  k is the Boltzmann constant.