Air taxi Captain reported they smelled smoke in the flight deck during descent. They ran the QRH and decided to return to the departure airport.

Narrative

While beginning the decent into ZZZ we left 16000 for 5000. At a 1500 ft per min decent with moderate turb we started to smell smoke in the cockpit. At first it was burnt rubber then burning electrical. I pulled out the QRH and read through the smoke checklist. O2 was used and I followed the checklist. Once we leveled off there was no new smell. Upon landing we called base and advised of the event. We spent a few minutes trouble shooting and came to the conclusion that it was potentially the left windshield heat. We made the decision to MEL and RTB (Return to Base). The med crew was advised of the options and we opted to RTB. We returned to base at fl210 there was no ice and we remained clear until the decent where it was raining mod to heavy. There were no reports of icing. During the flight I tested the windshield heat and there was no smell. 10 min from base in the decent leaving 10000 the smell returned. Again we followed the QRH. The smell was worse and all three crew smelled it. On the RNAV XX into ZZZ1 smoke was seen on the RH (Right Hand) footwell. It was described as white/grey. I saw haze and smelled smoke. I advised ATC and out of an abundance of caution we requested priority handling. We landed safely and the smell dissipated. There were no other indications of life threating emergency.