Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 150; Flight Crew Type 10000
Situational Awareness; Communication Breakdown
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification
Human Factors
We briefed up 26L; new ATIS gave us 1L; and then ATC offered 1R which we accepted.  We briefed the visual to 1R backed up by the RNAV (GPS) 1R. ATC gave us vectors for the visual; and cleared us to intercept the 1R final. ATC kept stepping us down on final and we couldn't quite see the runway.  Our clearance was to 6;600 feet.  As we continued inbound on final; as the Pilot Monitoring; I noticed we were getting slightly high on the VNAV final.  I mistakenly thought we had been cleared the visual and dialed the altitude window down to zero. The FO said he didn't think we were cleared the visual yet; but the command bars signaled him to descend; so he slowly descended; with the terrain in sight; and I queried the Controller. We leveled off at 6;000 feet at 10 miles with the terrain in sight. ATC said our clearance was to 6;600 feet and asked if we had the terrain and airport in sight.  Now; we could see both terrain and airport and were cleared the visual. We flew the visual; backed up by the RNAV (GPS) 1R and landed uneventfully at LAS.As Pilot Monitoring I should have confirmed cleared the approach before dialing the altitude window to zeros. Also; when getting high on final I should have asked ATC for lower.
Air carrier Captain reported an altitude deviation during approach to LAS resulting in a CFTT.  The crew mistakenly believed they had been cleared for a visual approach which contributed to the event.