The current-voltage characteristics of several different types of material with partially metallic conductivity show similar nonlinearities. We analyze the current-voltage characteristics and their temperature dependence for metallic single-wall carbon nanotubes (SWNTs) suspended between electrodes and show their consistency with calculations for fluctuation-assisted tunnelling and activated conduction. We also make a comparison with earlier data on the current-voltage characteristics of networks of SWNTs and a SWNT rope. (c) 2005 Elsevier B.V. All rights reserved.