Single wall carbon-nanotubes in tight-binding approximation are one of the simplest nontrivial theoretical models with physical relevance that can be used for studying quantum mechanical transport mechanisms at the molecular scale. Based on this this model, we examine the interplay of disorder with external magnetic fields, leading to signs of weak localization. As expected, weak localization is enhanced in the energy regions with high density of states. We quantify the resulting energy dependent mean free path in relation to the sample size and the strength of the disorder. ``Coating wideband leads'' are introduced as a novel approach to model realistic contacts as they are found in experiment, without adding much computational complexity.