An electron flying through a gas is like a swimmer trying to cross a crowded pool. Each collision with an atom changes its course and speed. In dense argon gas, there are so many collisions that a simple description no longer works.
Physicists measured electron mobility — how easily electrons drift under an electric field. They varied gas density, temperature, and field strength. The results confirmed their previous model, which accounts for multiple scattering.
It's like a swimmer in a pool who not only collides with nearby swimmers but also with waves from everyone else. Only by accounting for all these interactions can you accurately predict how quickly they'll reach the edge.
Argon is an inert gas, but its atoms still strongly influence the motion of free electrons due to quantum scattering effects.