1. Cr Research Group, Department of Physics, University of Johannesburg, Auckland Park, P.O. Box 524, 2006 Johannesburg, South Africa
2. Nano Scale Transport Physics Laboratory, School of Physics, University of the Witwatersrand, Private Bag 3, WITS 2050 Johannesburg, South Africa
3. CIRIMAT, Université de Toulouse, CNRS, INPT, UPS, UMR N°5085, Université Toulouse Paul, Sabatier, Bât. CIRIMAT, 118, route de Narbonne, 31062 Toulouse Cedex 9, France