Bulk silicon, the bedrock of information technology, consists of the deceptively simple electronic structure of just Si Si c bonds. Diamond has the same lattice structure as silicon, yet the two materials have dramatically different electronic properties. Here we report the specific synthesis and electrical characterization of a class of molecules, oligosilanes, that contain strongly interacting Si Si a bonds, the essential components of the bulk semiconductor. We used the scanning tunneling microscope-based break-junction technique to compare the single-molecule conductance of these oligosilanes to those of alkanes. We found that the molecular conductance decreases exponentially with increasing chain length with a decay constant beta = 0.27 +/- 0.01 angstrom(-1), comparable to that of a conjugated chain of C=C pi bonds. This result demonstrates the profound implications of sigma conjugation for the conductivity of silicon.