  We studied flux pinning in exfoliated FeTe$_{0.6}$Se$_{0.4}$ thin-film
devices with a thickness $d$ from 30 to 150 nm by measuring the critical
current density $J_{\mathrm{c}}$. In bulk FeTe$_{0.6}$Se$_{0.4}$, the flux
pinning has been discussed in the framework of weak collective pinning, while
there is little knowledge on the pinning mechanism in the thin-film region.
From the thickness $d$ dependence of $J_{\mathrm{c}}$ at a fixed magnetic field
of 1 T, we found that the strong pinning is dominant below $d \approx 70$ nm,
while the weak collective pinning becomes more important above $d \approx 100$
nm. This crossover thickness can be explained by the theoretical model proposed
by van der Beek $\textit{et al}$ [Phys. Rev. B. ${\bf 66}$, 024523 (2002)].
