In general, the diffusion coefficient is roughly equal to the product of the velocity at which a particle moves between collisions, and the mean free path between collisions. In the case of a plasma, the velocity at which particles move is the thermal velocity
where is the Boltzmann constant, is the temperature and is the mass of the particles. The mean free path can be estimated in the following way. If we assume only interaction between neighbouring particles, and we imagine particles with charge flying next to each other with impact parameter , then the impact parameter at which the deflection angle is of order unity is
The mean free path for such an interaction is
where is the particle number density. The diffusion coefficient is therefore
We note that the assumption of interaction from nearest neighbours is not entirely justified. At a larger distance the electric field decreases on the one hand, but the number of particle increases, so that different radii have similar contribution, and this introduces a logarithmic correction to cross section.