Can you use Akali rouge assasin during an open battlefield and not conquering?

Yes. Rogue Assassin's [Action] ability can be activated during the open (non‑combat) showdown at the contested battlefield, and moving your unit back to base prevents the Conquer.

When your unit enters an open battlefield, it applies Contested and a Non‑Combat Showdown opens — you, as the player who applied Contested, gain Focus, and in Showdown Open state you may activate abilities with [Action]. Your unit there counts as "a friendly unit in a showdown," so it's a legal target. Once the ability resolves and the unit is at your base, a Cleanup runs, no units remain at the battlefield, control is never established, and Contested is removed — so no Conquer occurs (Rule 348.2, per FAQ #11800). One condition: the ability only works if it's your turn, which it will be since you made the move.