Electronic transport measurements of single, systematically varied 4,4'-bis(mercaptoalkyl)-biphenyl derivatives (MABP) are performed in a controlled test-device. The molecules are composed of a central biphenyl unit (BP) carrying two mercaptoalkyl substituents with different chain lengths (m, n = number of CH(2)-units), in the para-position of the BP unit. The total length of both spacers is m + n = 10. The molecular conductance of these individual MABPs deposited on Au (111) substrates is studied using STM-based break junctions. It is shown that the molecular conductance depends on the relative position of the BP unit within the molecule. In the case of the symmetric derivative 5BP5 a value of 0.07 +/- 0.01 nS is obtained, while for 1BP9 the molecular conductance is doubled and a value of 0.17 +/- 0.03 nS results. This relatively high value of conductance for the single Au(tip)-1BP9-Au(substrate) junction is attributed to an increased coupling of the BP unit to the adjacent electrode, i.e. the STM-tip or the Au-substrate. We address the role of the specific contact situation (-S-Au) and of the position of the electrically active molecular moiety and thus come to a deeper understanding of the electronic transport properties of 4,4'-bis(mercaptoalkyl)biphenyl derivatives.