You can use Unity’s rigidbody object to modify the drag property, which is used to modify the rate that an object slows down when not acted upon by outside forces.
Either that, or if you wanted to get down and dirty, simply do
velocity = velocity * 0.979;
every update tick when you would like to slow the object. Adjust the numerical value between 0 and 1 to achieve your desired rate of deceleration.
Drag is one of the most difficult things to model accurately. There are many different equations we use to model drag in the real world (it gets extremely complex) but typically for real time simulation in games, highly simplified versions are used. In the examples provided by this discussion, you can see that some people (and most physics engines) model drag using a quadratic value. This is perhaps more realistic, but not required to achieve the results you may be looking for.
I believe the equation for the force of drag they teach in beginner physics classes is typically Fdrag = -v * b where b is some constant value. This is essentially what you already had, but is a less desirable solution in games. In the end, choose the method that feels best for your project.