An approach to computing conductances of tunnel junctions within the framework of the Landauer-Büttiker theory for electronic transport is introduced and formulated for the Korringa-Kohn-Rostoker (KKR) method for electronic-structure calculations. After a general introduction to the idea behind the approach, tests and comparisons with other methods, namely a 'transmission of Bloch-waves' approach and an approach based on the Kubo-Greenwood formula for the conductivity tensor, reveal a high accuracy and robustness of the proposed method, thus proving its suitability for state-of-the-art computations of spin-dependent ballistic transport. Based on Green functions, it is flexible and can easily be implemented in present KKR computer codes.