| Research studies on systems with reduced size and dimensionality have attracted great attention for the past two decades. This is mainly driven by industrial development, which demands the fabrication of new, small, well-controlled devices as well as the desire to understand quantum effects which manifest in these small structures.; In this thesis we theoretically investigate quantum coherent transport properties of nano structures in the form of molecular electronic systems. Our approach is based on Landauer-Büttiker transport theory. However, the details of the method depend on the interaction complexity.; We have carried out detailed analysis on finite length carbon nanotubes based magnetic tunnel junction using tight binding atomic model and Green's function approach. This device shows clear spin valve effect even when contacted with the same ferro-magnetic material with a long spin scattering length. In addition to this, transport at the atomic level is highly affected by the molecular states resulting in conductance oscillation of finite size arm-chair carbon nanotube as a function of its length.; When short carbon nanotubes are weakly contacted to external leads, they act as quantum dots with strong interaction at the molecular scale. To analyse these systems, we have developed a many-body wave function formalism which include spin degeneracy. This approach clearly shows the strong dependence of the device electronic response on the number of electrons already inside the tube.; Finally, we have carried out ab initio analysis based on Density Functional Theory within Local Density Approximations to investigate the current-voltage (I-V) characteristics of various gold nanowires. Our results demonstrate that transport properties of these systems crucially depend on the electronic properties of the scattering region, the leads, and most importantly the interaction of the scattering region with the leads. For ideal, clean Au contacts, the theoretical results indicate a linear I-V behavior. However, when sulfur impurities exist at the contact junction, nonlinear I-V curves emerge due to a tunnelling barrier established in the presence of the S atom. The most striking observation is that even a single S atom can cause a qualitative change of the I-V curve from linear to nonlinear.; Our theoretical results were compared to experimental data, qualitative and sometimes quantitative understanding of the experiments are obtained. |
