Particle transport across braided magnetic surfaces is studied by using simple field and collision models. Diffusion along and across field lines is taken into account. A general variational principle is established. It provides a useful guide for numerical computations. In a special case an explicit formula for the diffusion coefficient is obtained. Several diffusion regimes are clearly identified, and their domain of validity is determined. A new form of the diffusion coefficient has to be introduced in the long‐mean‐free‐path regime.