retain momentum with characterController.Move

     When moving the character controller with input everything looks great. i can move on ground or in air. but when the button isn't pressed the character controller immediately stops moving. i am looking for an effect similar to the rigidbody's public method "addrelativeforce" along with it's drag, although i am adamant on not using one. 
  my goal is to get the character to slow to a stop in the direction it was last going. here's the bulk of it:

     void PlayerMovement()
     {
 
         isGrounded = controller.isGrounded;
         float hor = Input.GetAxisRaw("Horizontal");
         float ver = Input.GetAxisRaw("Vertical");
         direction = new Vector3(hor, 0, ver);
         direction = transform.TransformDirection(direction);
         controller.Move(Vector3.ClampMagnitude(direction, 1f) * speed * Time.deltaTime);
 
  
 
     }