In GaAs/InAs core/shell nanowires, comprising a tubular conducting shell, interference phenomena observed under an axial field and originating from closed-loop states encircling the insulating core, provide an ideal platform for superconducting quantum devices that utilize effects such as Aharonov-Bohm or Altshuler-Aronov-Spivak-type conductance oscillations. Both effects are different in nature with respect to phase rigidity because of interference of non-time-reversed or time-reversed paths, respectively. Since their occurrence is largely governed by averaging effects, which depend on sample dimensions and the transport regime, we present a systematic study of flux-periodic oscillations of phase-pure zinc-blende GaAs/InAs core/shell nanowires as a function of gate voltage for samples with different contact separation lengths. Our analysis shows that with increasing contact separation length, averaging effects result in gradual reduction of h/e-periodic Aharonov-Bohm-type oscillations, while the h/2e-periodic Altshuler-Aronov-Spivak oscillations and its h/4e-periodic higher harmonics are enhanced. The additional phase rigidity seen in the h/3e-periodic oscillations is attributed to phase rigidity propagating from the neighbouring lower harmonics. Our tight-binding transport simulations on nanowires of different lengths which contain only a few scattering centers confirm the experimental observations regarding the different harmonics and their phase rigidity. Together, our experimental and simulation findings indicate quasi-ballistic transport with persistent Aharonov-Bohm-, and phase-rigid Altshuler-Aronov-Spivak-type oscillations despite few scattering centers. (2026-06-02)