The mechanical strength of highly strengthened Cu-based conductor materials is strongly related to their microstructure, which in turn is a consequence of the history of processing. Hence, the strength is understood in terms of solid solution, precipitation, grain boundary as well as work hardening. However, there are limitations for strengthening the material by the use of these mechanisms and retaining a reasonable conductivity. Twin boundaries have been shown to strengthen thin films without reducing the conductivity. This gives rise to investigating the role of the stacking fault energy (SFE) with respect to deformation in order to flatten the path to a 'twin-boundary strengthened' conductor. During deformation the SFE controls the possibility of storing deformation energy in the form of dislocations and deformation twins. This is additionally affected by the temperature since dislocation slip is less pronounced at lower temperatures, thus enhancing deformation twinning. The onset of twinning is reflected by the starting point of stacking fault growth, which will be shown exemplarily for a FeMnNiCr steel with a low SFE. For this case, a model-compatible description of the mechanical behaviour is shown and a reasonable SFE is backward calculated upon tensile test data.