Impedance spectroscopy at high electric field strengths ranging from 73 kV/cm to 146 kV/cm was used to measure the low-field conductivity and third-order nonlinear conductivity of the ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C(6)mim] [NTf2]. The measurements were carried out at three different temperatures close to the glass-transition temperature T-g = 189 K. The third-order conductivity coefficient sigma(3)' was determined from higher-order harmonic contributions to the measured ionic currents. Apparent jump distances a(app)), derived from the low-field and the third-order dc conductivity were found to be similar to those derived for solid electrolytes. However, in contrast to various solid electrolytes, the frequency-dependent third-order conductivity sigma(3)' shows no change in sign in the crossover regime to dc to dispersive conductivity. (c) 2012 Elsevier B.V. All rights reserved.