  Disorder can have remarkably disparate consequences in superconductors,
driving superconductor-insulator transitions in ultrathin films by localizing
electron pairs and boosting the supercurrent carrying capacity of thick films
by localizing vortices (magnetic flux lines). Though the electronic 3D-to-2D
crossover at material thicknesses $d \sim \xi$ (coherence length) is well
studied, a similarly consequential magnetic crossover at $d \sim L_c$ (pinning
length) that should drastically alter material properties remains largely
underexamined. According to collective pinning theory, vortex segments of
length $L_c$ bend to adjust to energy wells provided by point defects.
Consequently, if $d$ truncates $L_c$, a change from elastic to rigid vortex
dynamics should increase the rate of thermally activated vortex motion $S$.
Here, we characterize the dependence of $S$ on sample thickness in Nb and
cuprate films. The results for Nb are consistent with collective pinning
theory, whereas creep in the cuprate is strongly influenced by sparse large
precipitates. We leverage the sensitivity of $S$ to $d$ to determine the
generally unknown scale $L_c$, establishing a new route for extracting pinning
lengths in heterogeneously disordered materials.
