We have studied the magnetoconductance in a topological insulator BiSbTeSe2 with different probe lengths. The magnetoconductance correction reduces by a factor of 2 when the probe length is comparable to the phase coherence length, Lφ, and the related weak antilocalization prefactor, α, reduces by a factor of 2. Lφ is independent of the probe length and follows the T-0.5, corresponding to the two-dimensional electron-electron interaction. α shows similar back-gate voltage dependence and Lφ is almost the same in both short and long channels. This indicates that the widely reported surface-to-bulk coupling is not the dominant mechanism of the α reduction. Moreover, non-saturating magnetoresistances are observed and coincided with each other in the short and long channels. The reduced α is deemed to be due to the quantum correction effect caused by the geometries and electrode distribution. The finding here will further the understanding of the transport properties of the topological insulators and unveil exotic quantum phenomena.