A charged particle is moving in a circular path in a magnetic field. A resistive force starts acting on the particle whose direction is opposite to the direction of  its motion and magnitude is directly proportional to its velocity. Now the particle starts moving in a spiral path, then

Radius r of particle in spiral path at any time when velocity is vr=mvqB;   ω=vr = qBm  ⇒  constantAngular momentum L=mvr=qBr2 decreases as r decreasesIf particle is moving in a plane then its velocity and acceleration will be in that plane. The force on the particle will also be in that plane. It means magnetic field should be perpendicular to that plane.Magnetic force on the particle = qvBResistive force on the particle = kv, where k is a constant As velocity decreases so net force acting on the particle will decrease.