Simulation results (black diamonds) for the parallel scattering mean free path in slab turbulence with adiabatic focusing as described by Eqs. (3)and (4). The focusing length is chosen as L/ℓ_slab = 10 (upper panel) and L/ℓ_slab = 1 (lower panel). Furthermore, the two analytical results are shown as open diamonds for the quasi-linear result from Eq. (9)and as gray triangles for the nonlinear result from Eq. (10). For comparison, the parallel mean free path is shown also for a homogeneous background magnetic field (gray circles). The magnitude of the turbulence is (δB/B₀)² = 0.1.