please explain this statement in detailed and answer following questions : "The Higuchi model is often used to describe Fickian diffusion to determine whether it is a rate-limiting step".

a) what is Fickian diffusion? how higuchi model can describe fickian diffusion?

b) how can higuchi model be used to determine rate - limiting step?

give example to this when you explain please. I really don't understand anything from this statement. please help. thanks so much.