I am measuring the resistivity of the sample using the four probe measurement. My electrodes are rectangular (1mm*5mm )shaped platinum (50nm thickness) deposited on top of the sample separated by 1mm width . Considering the penetration depth of the sample, Which area should i choose as the Area for the resistivity calculations. Should i use the entire base area of the sample (could have shorter current penetration depth), Or should i use the Area of the current electrode ( considering the current Penetration depth ) for accurate calculation !!!