  We have measured the surface impedance of thick superconductors in the mixed
state over a broad 2 kHz - 20 MHz frequency range. The depinning cross-over is
observed; but it is much broader than expected from classical theories of
pinning. A striking result is the existence of size effects which invalidate
the common interpretation of the low-frequency surface inductance in terms of a
single penetration depth. Instead, a two-mode description of vortex dynamics,
assuming free vortex flow in the bulk and surface pinning, accounts
quantitatively for the spectrum of the complex apparent penetration depth.
