  We investigate the effects of magnetic and electric fields on electron
wavefunction interactions in single walled carbon nanotube bundles. The
magnetoresistance measurements performed at 4.2K and the dependence of the data
upon the electric field, obtained by varying the bias current through the
samples, reveal good agreement with weak localization theory. Recording
current-voltage characteristics at different temperatures we find an ohmic
non-ohmic transition which disappears above 85K. Conductance vs temperature
measurements are also well explained in the framework of weak localization
theory by the predicted temperature dependence of the electric
field-conditioned characteristic length. This length results equal to the
average bundles diameter just at T{\backcong}85K, indicating that the observed
conductance transition is due to a 2D-3D crossover.
