  The field-dependent ion transport in thin samples of different glasses is
characterised by means of nonlinear conductivity spectroscopy. AC electric
fields with strengths up to 77 kV/cm are applied to the samples, and the
Fourier components of the current spectra are analysed. In the dc conductivity
regime and in the transition region to the dispersive conductivity, higher
harmonics in the current spectra are detected, which provide information about
higher--order conductivity coefficients. Our method ensures that these
higher--order conductivity coefficients are exclusively governed by
field--dependent ion transport and are not influenced by Joule heating effects.
We use the low-field dc conductivity $\sigma_{1,dc}$ and the higher--order dc
conductivity coefficient $\sigma_{3,dc}$ to calculate apparent jump distances
for the mobile ions, $a_{\rm app}$. Over a temperature range from 283 K to 353
K, we obtain values for $a_{app}$ between 39 \AA $ $ and 55 \AA . For all
glasses, we find a weak decrease of $a_{\rm app}$ with increasing temperature.
Remarkably, the apparent jump distances calculated from our data are
considerably larger than typical values published in the literature for various
ion conducting glasses. These values were obtained by applying dc electric
fields. Our results provide clear evidence that the equation used in the
literature to calculate the apparent jump distances does not provide an
adequate physical description of field-dependent ion transport.
